1. Field of the Invention
The invention relates to an optical device, and more particularly to an optical device having an E-mode polarizer to enhance brightness and viewing angle property and prevent color shift.
2. Description of the Related Art
Of all panel display types, only liquid crystal display (LCD) uses linearly polarized light to create brightness, darkness, and grey level. In general, a polarizer is employed to transform incident light provided by a backlight module into the linearly polarized light. In detail, as shown in FIG. 1, a conventional optical device has a lamp 10 that provides incident light to one side of a diffuser plate 12. After the light enters a dichroic polarizer 14 with unidirectional light absorbing nature, the linearly polarized light in one direction is absorbed, and the linearly polarized light in another direction passes to accompany twisted liquid crystal molecules, causing a variation in brightness to display plentiful images. Since the dichroic polarizer 14 absorbs half the incident light, however, reducing a large part of the source luminescence, the brightness of the conventional LCD is about 4˜6% of the brightness of the lamp 10. Accordingly, before the light enters the polarizer, an advance treatment on the incident light is needed to generate a specific linearly polarized light that completely passes through the polarizer, resulting in enhanced efficiency of the light source and subsequent increased luminescence of the LCD.
In order to solve the above-described problems, some research suggests that the backlight module directly generates the linearly polarized light, but this idea is difficult to realize. In other research, a reflective polarizer that itself does not absorb light is employed to significantly enhance luminescence of the LCD. The reflective polarizer, providing a light recycling mechanism, comprises a retro-reflective type and a cholesteric type. The former reflective polarizer deals with the linearly polarized light, but has disadvantages of complicated processes and difficulties in mass production for manufacturing more than 800 layers' structures. The cholesteric reflective polarizer deals with the circularly polarized light, and has advantages of simplified process and facility in mass production only if an appropriate liquid crystal control technology is utilized.
FIG. 2 is a schematic diagram of a conventional optical device having a cholesteric reflective polarizer 16. The cholesteric reflective polarizer 16 has a cholesteric liquid crystal layer 15. When the cholesteric liquid crystal molecules are arranged as a planar alignment, the helix structure 15A makes the optic axis perpendicular to the substrates. Therefore, the cholesteric liquid crystal layer 15 can separate the unpolarized incident light into right-handed/left-handed circularly polarized light, in which the circularly polarized light having opposite handness to molecular helix can be transmitted and the same-handed circularly polarized light can be reflected. Thereafter, using a reflective surface under the diffuser plate 12, the reflected-type light can be easily transformed into the transmitted-type light that can pass through the cholesteric liquid crystal layer 15. Concretely, a right-handed light 19CR is transformed into a left-handed light 19CL, and then the left-handed light 19CL can pass through the cholesteric liquid crystal layer 15 to becomes unitary-handed type light 19CU. Thus, all of the light from the lamp 10 can be transformed into unitary circularly polarized light, that is, there will be two-time light theoretical. Moreover, when a ¼ wave phase retardation plate 18 is cooperated with the cholesteric reflective polarizer 16, the circularly polarized light 19CU can be further transformed into the linearly polarized light 19N. Finally, the incident light provided by the lamp 10 is completely transformed into the linearly polarized light 19N passing through the polarizer, thus increasing the luminescence of the LCD.
The cholesteric reflective polarizer 16 has an optical anisotropic nature that changes the color related to the reflective light in accordance with the change in the viewing angle. This color shift in off-axis is explained by Bragg reflection theorem: nopcos θ<λ<nepcos θ, wherein λ indicates the main wavelength of the reflected light from the cholesteric liquid crystal layer, no and ne indicate the ordinary and extraordinary refractive indexes of the cholesteric liquid crystal respectively, p indicates a helix pitch corresponding to a 2π molecular rotation, and θ indicates a viewing angle. When the viewing angle is increasing, the wavelength of the reflected light is decreasing to cause color shift. This color shift in off-axis is also explained by a qualitative analysis as the follows. The helix structure 15A of the cholesteric liquid crystal can be regarded as a discotic molecule as viewing the superimposed structure from a normal direction. When the viewing angle gradually increases, the discotic molecule with symmetrical profile gradually changes into an elliptic molecule with long axis and short axis, causing different phase shifts in accordance with different wavelengths.
In order to compensate for the color shift, several methods have been developed as follows. In U.S. Pat. No. 5,737,044 and U.S. Pat. No. 5,825,444, Philips company discloses a broadband cholesteric polarizer whose surface of maximum helix pitch faces the light source. Also, in WO97/19385, Nitto Denko discloses a cholesteric polarizer whose surface of minimum helix pitch faces the radiation source. Unfortunately both rely on experimental results, not theoretical calculations, and it is realized that the compensation function is lost at a large viewing angle. Additionally, in U.S. Pat. No. 6,061,108, Sharp company discloses a multi-layer structure including pairs of cholesteric layers and compensating layers to allow the effective bandwidth of the polarizer to be increased. The cost of this structure is, however, too high to effectively utilize. Finally, U.S. Pat. No. 6,088,079 discloses an optical element comprising a cholesteric liquid crystal layer and a retardation compensating plate that has a value Nz=−1.2˜0.2.